1. Field of the Invention
The present invention relates to vanadium additions to steel and more particularly to an essentially oxygen-free, low-carbon vanadium nitride additive and to an improved process for preparing the same.
2. Description of the Prior Art
It has already been proposed to produce vanadium nitride by ammonia reduction of ammonium metavanadate according to the following reaction: EQU 21NH.sub.4 VO.sub.3 +14NH.sub.3 .fwdarw.21VN+63H.sub.2 O+7N.sub.2
However, even with excess ammonia, the reaction is difficult to bring to completion, i.e., to an essentially oxygen-free product. Long reaction times, large excesses of ammonia, and fairly small charges are needed to achieve essentially pure vanadium nitride.
For example, Roubin and Paris in C.R. Acad. Sci. Paris 260, pages 3088-91 (1965) treating ammonium metavanadate samples for 20 hours in a current of ammonia and hydrogen achieved vanadium nitride products containing 2.8 wt.% oxygen, 0.6 wt.% oxygen, and 0.1 wt.% oxygen at temperatures of 700.degree. C., 900.degree. C. and 1000.degree. C., respectively. From an economic standpoint, it is undesirable to perform the nitriding for long periods of time with large excesses of ammonia which are needed to bring the oxygen level below 1.0 wt.%. Although high temperatures increase the rate of oxygen removal, they also increase the rate at which ammonia is catalytically decomposed to nitrogen and hydrogen, which as a mixture are much less efficient nitriding agents than ammonia.
Guidotti, Atkinson and Kesterke in U.S. Bureau of Mines, Report of Investigations 8079, (1975) have shown that the initial rates of reaction of ammonia with V.sub.2 O.sub.3 in various materials begin to decline above about 800.degree. C. for nonmetallic materials of construction, and above 700.degree. C. for metallic materials of construction. However, essentially oxygen-free vanadium nitride could not be produced below 1000.degree. C. in the nonmetal reactors and below about 900.degree. C. in metallic reactors, even with a substantial stoichiometric excess of ammonia for small charges (e.g., 5 gram) of V.sub.2 O.sub.3. Fairly rapid oxygen removal from V.sub.2 O.sub.3 by NH.sub.3 in an Al.sub.2 O.sub.3 reactor at 800.degree. C. was achieved, however, but the final product contained 3.8 wt.% oxygen. A product containing less than 1.0 wt.% oxygen is desirable for use in the steel industry.
Another known approach to obtaining a low-oxygen vanadium nitride is by carbon reduction of an oxide such as V.sub.2 O.sub.3 in a nitrogen atmosphere according to the following reaction: EQU N.sub.2 +V.sub.2 O.sub.3 +3C2VN+3CO
An excess of carbon over that for the reaction is required to produce low-oxygen products. This results in products containing excess carbon, i.e., materials for which some carbon is substituted for nitrogen. Low-carbon products can be produced by very high temperature operation in a nitrogen atmosphere, e.g., 1800.degree. to 2000.degree. C., where close to the stoichiometric amount of carbon for the above reaction can be used. However, even this stoichiometric amount represents a high usage of the rather expensive thermal grade carbon required.
Another approach to obtaining low-oxygen, low-carbon vanadium nitride using the stoichiometric carbon of the above reaction is taught in U.S. Pat. No. 4,040,814 to R. F. Merkert. In this patent, the V.sub.2 O.sub.3 and carbon mixture is subjected to a cyclic vacuum-nitrogen-vacuum treatment at temperatures of between 1100.degree. and 1500.degree. C. However, up to five cycles of alternate vacuum and nitriding treatment may be required to produce a vanadium nitride product containing less than 2 wt.% oxygen and carbon in the aggregate.